Embodiments of the invention concern the field of communication systems, for example, mobile communication systems comprising a plurality of base stations serving respective mobile units. More specifically, embodiments of the invention refer to a communication system and a method for directly allowing a communication among respective nodes thereof, for example among respective base stations, as well as to a node or base station and an optical multiplexer/demultiplexer device of such a communication system.
In a communication system a plurality of nodes and a central switching device are coupled via a backhaul access network for exchanging signals between the nodes. However, there may be circumstances that need the exchange of information directly between the respective nodes, for example between respective base stations of a mobile communication system. For mobile communication systems, for example, coordinated multi-point (CoMP) schemes have been discussed in 3GPP (third generation partnership project) due to their potential to improve user data rates by allowing different nodes to participate in the transmission and the reception of user data. Examples of such schemes are discussed by M. Sawahashi, Y. Kishiyama, A. Morimoto, D. Nishikawa and M. Tanno, “Coordinated multipoint transmission/reception techniques for LTE-advanced,” IEEE wireless communications, vol. 17, issue 3, pp. 26-34, 2010. CoMP schemes need the exchange of user data as well as the exchange of cell information, for example channel state information (CSI), through mobile backhaul networks so that the achievable performance enhancement strongly depends on the mobile backhaul capability, as is discussed by D. Samardzija and H. Huang, “Determining backhaul bandwidth requirements for networks MIMO,” EUSIPCO, Glasgow, August 2009. In many cases, signal and data exchanges for a CoMP transmission are needed between neighboring nodes or base stations (eNBs), since adjacent nodes usually yield the most significant influence on interference and received signal power level of mobile users. For such a communication, the X2 interface defines a logical interface between two nodes (eNBs) and is used for exchanges/transmissions to support the CoMP transmission. The X2 interface as defined in the 3GPP standard is not a physical interface, but a logical interface which depends on the specific hardware implementation of the actual physical interface.
FIG. 1 shows an example of physical and logical X2 interfaces in a mobile backhaul access network. The network comprises a central switching unit 100 and a plurality of base stations 102a, 102b servicing respective mobile units, e.g. mobile unit 104. FIG. 1 is just a schematic representation and in reality a communication system will comprise a plurality of mobile units and also a plurality of base stations, i.e. more than two base stations. The central switching unit 100 and the respective base stations 102a and 102b are connected via a mobile backhaul access network 106. The network 106 may be an optical network which comprises an optical multiplexer/demultiplexer device 108 for combining/splitting signals transmitted via the network 106. The network 106 comprises a connection 110 (e.g. an optical fiber) between the central switching unit 100 and the optical multiplexer/demultiplexer device 108 and a plurality of branches 1121-112n (e.g. optical fibers). The base station 102a is connected to the central switching unit 100 via the connection 110 and the branch 1124, and the base station 112b is connected to the central switching unit 100 via the connection 110 and the branch 1123. The mobile unit 104 is provided for a CoMP transmission, i.e. the unit 104 communicates with the base station 102a via a first channel 114a and with the base station 102b via a second channel 114b. This communication needs the exchange of information, e.g. the exchange of signals and data, between the neighboring base stations 102a and 102b. The base station 102a is assumed to be the servicing base station or servicing eNB, and the base station 102b is assumed to be the cooperating base station or cooperating eNB. The exchange of information between the base stations 102a and 102b needs the above-mentioned X2 interface which is a logical interface schematically shown in FIG. 1 at reference sign 116. The logical X2 interface is realized via a physical interface, the physical X2 interface, shown in FIG. 1 at reference signs 118a and 118b. The physical X2 interface has a first component 118a extending between the servicing node 102a and the central switching unit 100 and a second component 118b between the central switching unit 100 and the cooperating node 102b. For transmitting data between the nodes 102a and 102b using the logical X2 interface for supporting the CoMP transmission of the mobile unit 104, it is needed to transmit the actual data from the servicing node 102a via the first component 118a of the physical X2 interface to the central switching unit 100 and from the central switching unit 100 back to the cooperating node 102b via the second component 118b of the physical X2 interface 118b. 
The physical X2 interface 118a, 118b is realized using the network 106 and the S1 traffic and the X2 traffic share the resources of the network 106. While this may minimize or reduce hardware costs, it results in the problem that the latency and the capacity of the X2 interface may not fulfill the requirements for information exchange in accordance with the CoMP scheme. Implementing the X2 logical interface in a way as depicted in FIG. 1 incorporates a large delay associated with the OEO conversion and the packet processing and due to the long fiber transmission via the links 118a and 118b. In addition, a large processing burden for the central gateway (the central switching unit 100) exists due to the additional OEO conversion. Further, since the X2 interface shares the physical link with the S1-U interface only limited bandwidth is available.
So far, the implementation of the X2 interface as depicted in FIG. 1 has been accepted, since the LTE release 8 (LTE=long-term evolution) only necessitates that the latency of an X2 interface needs to be in the range of 20 ms maximum with a typical average of 10 ms, which is not a problem in the implementation shown in FIG. 1. The reason for this is that in the practical implementation the X2 communication between respective nodes, the X2 inter-eNB communication was limited, for example, to data forwarded for a handover and for a control plane support for the radio resource management. Such an implementation does not need a low latency in the range of only a few ms as it is needed by a CoMP transmission. However, when implementing a CoMP transmission the latency and the limited capacity of the X2 interface realized in a way as shown in FIG. 1 form a bottleneck for CoMP, since, in general, a CoMP transmission needs less than a few ms latency and a true Gbps traffic for the inter-eNBs communication (the communication between the respective base stations). The exact values depend on the actual CoMP technique realized (see e.g. D. Samardzija and H. Huang, “Determining backhaul bandwidth requirements for network MIMO,” EUSIPCO, Glasgow, August 2009 and T. Pfeiffer, “Converged heterogeneous optical metro-access networks,” ECOC 2010, Turin, September 2010).
Besides the CoMP schemes also other aspects within a mobile communication network benefit from a direct communication link between respective base stations. For example the increased frequency handover in a network having a smaller cell size in accordance with the LTE-advanced standard will need more information to be exchanged through the X2 interface. For this exchange of large amount of information a direct communication link, i.e. a direct X2 physical interface, between respective base stations may also be interesting. Thus, the use of a direct communication link for the X2 link may not only be of interest for a CoMP transmission, but also for transmission of other data between respective, for example, neighboring nodes or base stations.
For addressing the above problems conventional approaches are known implementing a direct communication link between eNBs for realizing the X2 physical interface instead of implementing the interface using the mobile access network in a way as shown in FIG. 1. One conventional approach would be to provide additional signal lines directly connecting the base stations, e.g. providing additional fiber links between the base stations shown in FIG. 1. However, it is not practical to deploy additional fiber links for the X2 interface due to the associated costs.
Another conventional approach in shown in FIG. 2 and provides a direct communication link between the respective base stations via a wireless communication link, for example by providing microwave wireless backhaul links. Each of the base stations 102a and 102b is provided with a microwave transceiver 120a, 120b allowing for a wireless communication between the respective base stations 102a and 102b, for example via microwave links operating at 7, 10, 13, 28 or 38 GHz. The direct communication link 122 between the base stations 102a and 102b provides the physical X2 interface allowing for the direct exchange of information in accordance with a logical X2 interface. The communication link 122 allows for a bandwidth of more than 400 Mbps and a latency of about 0.5 ms. However, providing the X2 physical link by installing a microwave/millimeter-wave point-to-point link between two base stations is a very expensive solution as it needs a huge number of additional hardware for the wireless backhauls to cover all nodes. Also an additional license for the frequency band used is needed. Also, the link typically does not offer as much link quality as the backhaul fiber-optic link due to its susceptibility to the environment, e.g. the weather conditions.
Yet another known approach is the use of a TDM-PON (TDM=times division multiplex; PON=passive optical network) having X2 physical links, as described by T. Pfeiffer, “Converged heterogeneous optical metro-access networks,” ECOC 2010, Turin, September 2010. A time-division multiplexing passive optical network (TDM-PON) having splitter boxes is used for providing the X2 physical links between the respective nodes. Thus, the drawback of the FIG. 1 embodiment, where the transmission goes through the access gateway is avoided. However, only broadcasting of signals is possible, i.e. no point-to-point communication is possible, as it is needed for a direct communication link between respective base stations or nodes (e.g. needed by the X2 interface). In addition, using the plurality of splitter boxes increases the costs and reduces the SNR (signal-to-noise ratio), which is a problem with regard to the transmission in the Gbps range. In addition, due to the splitting ratio the loss of a large amount of signal-to-noise ratio cannot be avoided thereby limiting the data rate in the X2 interface so that the TDM-PON cannot support more that 1-Gbps bandwidth per node. Further, a plurality of splitters is needed to cover all optical network units in one passive optical network system as well as a careful signaling to avoid collisions between different X2 communications.